Discrete visible and near-infrared luminescence of a beam of photoexcited helium clusters is reported. The emission lines are attributed to free helium atoms and molecules desorbing from clusters in electronically excited states. Depending on the excitation energy, various atomic and molecular singlet and triplet states are involved in the relaxation process. With increasing cluster size the intensity of molecular transitions becomes dominant. The temperature of ejected molecules could be estimated to $T_{\mathrm{vib}}\sim 2500\mathrm{K}$ and $T_{\mathrm{rot}}\sim 450\mathrm{K}$ and is much higher than that of the cluster itself.

• Received 24 March 1997

DOI:https://doi.org/10.1103/PhysRevLett.78.4371